I would think anything that eliminates the odor will remove the temptation to use the area again. Sounds like maybe the owner isn't cleaning the box often enough? If it would work in the space, I'd clean as well as possible and spread a heavy duty plastic tarp as large as will fit under the box and surrounding area (maybe add a 2nd box?) so at least accidents won't land on the floor again. I had good luck using puppy pee pads spread around randomly when I had a cat who stopped peeing in the box. I don't know if they might work for bm but worth a try.

Eta after cleaning and drying the area, you can also paint over raw wood with Kilz or similar to seal in the odors left.